Due to constraints of physical conditions, compared to the wired link, the wireless link in the mobile communication system has a low transmission rate, and a high bit error rate. In order to effectively make use of limited radio channel bandwidth resources, the Robust Header Compression (ROHC) is introduced. The main idea of the ROHC is to transparently compress and decompress information in the header of the packet between directly connected nodes by using information redundancy between service stream packets.
At present, the ROHC supports for compression and decompression of packet data packets of the types of Internet Protocol (IP)/User Datagram Protocol (UDP)/Real-time Transport Protocol (RTP), IP/UDP, IP/Encapsulating Security Payload (ESP). When the packet of IP/UDP/RTP type is compressed, a time stamp field in the RTP protocol is an important field to be compressed. At present, in the RFC3095 protocol, compression methods such as scaling compression, time-based compression and Self-Describing Variable-Length (SDVL) encoding etc. are provided.
The SDVL encoding method refers to representing a length of the encoded field using a number of high bits of the first byte. If the highest bit of the first byte is 0, it is indicated that the length of the encoded field is 1 byte, and the maximum compressible value is 127; if the highest two bits of the first byte are 10, it is indicated that the length of the encoded field is 2 bytes, and the maximum compressible value is 16383; if the highest three bits of the first byte are 110, it is indicated that the length of the encoded field is 3 bytes, and the maximum compressible value is 2097151; and if the highest three bits of the first byte are 111, it is indicated that the length of the encoded field is 4 bytes, and the maximum compressible value is 536870911.
The RFC3095 protocol defines a basic compression packet type and an extended compression packet type, and when the basic compression packet is insufficient to carry the corresponding compression information, an extended compression packet can be added behind the basic compression packet. The extended compression packet can be divided into extended type 1, extended type 2 and extended type 3.
When the basic compression packet carries an extended compression packet of type 3, the RFC3095 stipulates to store high bits of the time stamp field in the basic compression packet, and the low bits are performed with SDVL encoding and then are put into the extended compression packet. When the binary representation of the time stamp is more than 29 bits, and the field value stored in the extended compression packet begins with zero, it is needed to use up all storage space related to the time stamp in the basic compression packet and the extended compression packet when using the SDVL encoding. The current processing mechanism has the following defects:
There may be a plurality of bits of successive zeros in the extended compression packet, which wastes the storage space of the compression packet, has relatively low compression efficiency, and occupies more radio bandwidth resources.
A particular processing is needed when using the SDVL encoding in the extended compression packet.